This invention relates to a variable speed DC motor control module which can be employed for use with electric power tools such as power screw drivers and drills, and more particularly there is described a variable DC motor controller utilizing a zero displacement switch and associated electronic control circuitry.
The field of control devices for power tools is widespread. The prior art is cognizant of many devices which operate to control the speed of a rotational tool such as a drill bit, a screw driver shaft, and other such tools as well. In order to understand the type of prior art, a few definitions of concern will be explained. The majority of hand-held electric screw drivers today accomplish what is commonly called torque limiting by the use of a clutch. These devices decouple power not torque. Ideally, when such a device stalls, certain of the devices convert power into torque. To understand this, one must delve into the nature of the terms. Torque is a moment of rotational force expressed in pounds-feet or Newton-meters. Torque has no time component.
Torque, therefore, is only a component of what really drives the screw or drill. In very simple terms horse power is derived from torque times RPM. As one will ascertain, this term--namely, horse power does have a time component. This relationship is accurately portrayed in the workings of a common reduction gear. Ignoring other factors, 100 pounds-feet of torque at a 100 revolutions per second driving through a 10:1 reduction gear will output a 1,000 pounds-feet of torque at 10 RPS. This phenomenon is generally called "torque multiplication".
The term is apt because the horse power produced is identical: 100.times.100 or 1,000.times.10 still equals 10,000 units per second. Thus, dead lifting 550 pounds-feet per second equals 1 HP. This has several ramifications as applied to DC electric motors or other wound coil EMF devices. The most important is that at zero RPM the motor produces zero horse power and at any RPM up to its intrinsic maximum, an electric motor's horse power is directly related to its RPM.
The above is important because any electric motor has an intrinsic torque; a product of motor design constraints and not power or gearing variables. By far, the most important of these are the number, diameter and placement of the windings in the motor. The EMF produced by a given motor is governed mostly by the physical properties of its armature and stator. Only the rate of magnetic saturation is governed by electrical constraints. Voltage increases or decreases therefore only cause the motor to run faster or slower, but since the same EMF is produced, the same torque is produced. Since the motor spins at a different RPM the same torque produces more or less horse power. Only by producing a motor with variable coil parameters can one directly control the torque of the motor. As one will understand, this is extremely complex and very expensive. Thus, as one can see, it is the horse power and not the torque that does the real work in regard to DC motors.
The torque limiters which are employed on dedicated electric screw drivers or drills typically use a sprag (cone and cup) or dry plate friction-clutches preloaded by a spring to a user selected slippage point. These are real clutches and when stalled, they may actually behave as torque converters. The motor runs at a constant speed (and horse power) and when the resistance of the load which is the screw or tool being driven exceeds the clutches ability to hold, the load is partially or totally decoupled from the motor by slippage. The excess is wasted as friction generated heat and motor braking. It is critical to note that this process is not inherently smooth and what smoothness there is depends on a multitude of interrelated factors many of which depend on use, temperature, moisture, material hardness, penetrability, preload selection, and fastener type. These, of course, are just a few of the relevant factors. The friction surfaces may slide but they can and do also bounce against one another causing force to be applied as an oscillating load to the output.
The driven shaft experiences alternating full and no loads as the clutch grabs and then disengages. If the frequency of the bouncing is slow, it will be felt as a pronounced shuddering or even jumping of the tool. Users have experienced this in utilizing power drills as well as power screw drivers. Less expensive tools use an even less desirable alternative. Such tools employ planetary gear sets which disengage from one another by a floating preloaded ring gear. These types do not produce torque conversion even when stalled but rather something akin to ratcheting horse power. The effect is clutch like, but since they are not slippage devices, they cannot convert power into moment.
In any case, it is the primary operational disadvantage of all mechanical couplers. A grabbing clutch can be a very pronounced hazard especially at high shaft speeds with hard to start but easy penetrated materials like predrilled sheet metal. High initial preload and load slip are required to start the fastener, but once threading is complete, the load drops radically with a correspondingly rapid increase in shaft speed. This condition is worsened by the high multiplication gear boxes typical of these types of tools which have a great deal of rotating mass and hence much inertia.
The high preload on the clutch plates which is necessary to thread the screw now becomes an enemy. When the already threaded and now furiously spinning fastener bottoms, the tool will jump resulting in at least a momentary loss of contact. This usually causes stripped screw heads and certainly reduces the life of driver bits. This can be reduced substantially by modulating the motor speed with a variable resistance hand-operated trigger. However, this requires a great deal of skill on the part of the user and is not a successful solution. Thus, another way to overcome the situation is for the tool user to exert a large amount of pressure on the bit using his body weight and arm strength to counter the kick from the tool. Thus, as one can ascertain, soft fasteners as those fabricated from aluminum and so on often strip anyway due to the high inertia and so on.
Extensive industrial tools often go so far as to add complex centrifugal or overruning mechanisms to freewheel and to thereby decouple the tool when it kicks back. In any case, only at the point where the fastener ceases to rotate does torque itself become a factor. An ideal slipping clutch converts motor power into static load (torque) plus wasted heat. However, neither of these factors are the major contributors to stripped heads and bits. The trouble is that when the clutch either seizes unexpectedly on trigger release or bounces at a slow rate with the power on, many mechanical couplers not being true clutches cannot seize or slip accordingly. Hence, they operate only by bouncing constantly.
As one will also ascertain, the load applied to the screw during clutch or ring gear bounce is exactly analogous to that of a stalled motor operating at full torque for a percentage of the time. This percentage of time is equivalent to the engagement and disengagement ratio of the bouncing clutch. If this rate is rapid, slow human responses will perceive it as a vibration or may even ignore it. As will be explained, it is this particular aspect that the control device of this invention utilizes to good advantage.
The prior art is replete with many patents which operate to control the speed of machine tools or to otherwise stop the tool when excessive operating constraints are being applied to the tool.
Reference is made to U.S. Pat. No. 4,077,736 issued Mar. 7, 1978 to Hutchens et al. entitled DRILL SPEEDER FOR MACHINE TOOLS. This patent describes a centrifugal switch which is located in the tool holder and closes when the tool holder is rotated above a predetermined speed. The switch opens when the rotary speed of the tool holder falls below the predetermined speed.
See U.S. Pat. No. 4,292,571 issued on Sept. 29, 1981 to G. Cuneo entitled CONTROL DEVICE FOR CONTROLLING THE ROTATIONAL SPEED OF A PORTABLE POWER TOOL. This patent relates to a speed control device which automatically adjusts the speed of the motor in accordance with changes in the diameter of the tool bit being used. The diameter of the tool bit is a function of the rotational speed of the motor, and the device operates to produce an output signal which controls the firing of a thyristor which in turn controls the current supplied to the motor. Thus, in this manner the speed of the drill is controlled according to the diameter of the tool bit employed in the drill. This is typical of some of the above-described prior art devices.
See also U.S. Pat. No. 4,413,936 issued on Nov. 8, 1983 an entitled CONTROL DEVICE IN A MACHINE TOOL to G. Kuhlmann. This patent relates to a mechanical means which operates to control the drive of a machine tool to an optimum speed of rotation according to the tool which is used on the machine. The apparatus relates to hand-held drills, screw drivers and so on. This is one type of mechanical approach that has been described above and has the disadvantages of the above devices.
U.S. Pat. No. 4,418,765 issued on Dec. 6, 1983 to T. Mori et al. entitled POWER DRIVEN SCREW DRIVER WITH A TORQUE CONTROL relates to a screw driver which is motor driven and operates to determine the torque which is applied to the base of the tool whereby the detected torque is compared to the signal from a torque setting device which is indicative of a desired torque and with which the screw is to be tightened. The motor is energized accordingly so that the screw driver can be operated at the proper speed. There is an adjustable time delay circuit which is connected between the comparator and a switch to delay the signal from the comparator for a period of time which can vary depending on the material that the screw is being driven into. This patent is generally pertinent to some of the concepts to be described herein but has many disadvantages in regard to the way and the manner in which the signal is detected and employed.
U.S. Pat. No. 4,487,270 issued Dec. 11, 1984 to S. Huber entitled ELECTRIC TOOL, PARTICULARLY A HANDTOOL, WITH TORQUE CONTROL shows a pressure sensor which is built into the motor shaft and which provides a signal for controlling the speed of the motor in response to an axial thrust as to control or limit the torque of the motor.
U.S. Pat. No. 4,534,420 issued on Aug. 13, 1985 to R. Goldelius entitled ELECTRIC TOOL WITH TORQUE MONITOR shows an electric tool which operates in conjunction with a pressure sensor which sensor changes as a function of mechanical pressure exerted on the motor to control the torque or speed of the motor according to this pressure.
U.S. Pat. No. 4,540,381 issued on Sept. 10, 1985 to F. Hornung et al. entitled ROTARY ELECTRICAL TOOL WITH SPEED CONTROL ESPECIALLY DRILL shows means for the selective control of the speed of the tool such as drill or screw driver which is coupled to a motor. The speed of the tool is sensed by a contactless transducer which is coupled to the chuck and to the housing to provide speed signals which control the transmission ratio of the gears. As one can ascertain, this is a relatively complicated device and is of the type of device as described above in the background of art.
U.S. Pat. No. 4,745,557 issued on May 17, 1988 to G. Pekar et al. entitled MACHINE TOOL CONTROL SYSTEM describes a complicated feedback system which controls a machine tool which is capable of drilling through multi-ply work pieces where the multi-ply work pieces comprise different thicknesses and hardness. A the machine tool setting device sets a tool tip at a predetermined distance above a work piece by detecting when the machine tool intercepts an optical beam. The control system allows for the drilling of the multi-ply work pieces wherein the drill bit's translational and rotational velocity are controlled for optimization as a function of the type of material and the thickness of the material being employed.
As one can ascertain from the above, certain prior art devices are relatively complicated or do not generally operate to perform according to desired attributes necessary to properly and reliably control a tool bit associated with an electrical drill or electric screw driver as driven by a DC motor.
It is therefore an object of the present invention to provide a controller for a hand-held power tool as a screw driver or a drill which eliminates many of the above-described problems.
It is a further object of the present invention to provide an electronic controller for a hand-held power tool which controller employs a zero displacement switch which has no moving parts thus increasing the life of the tool as well as providing reliable operation.